KILN CONNECTION: Pulse Perfect

October 1, 2009

Figure 1. Initial pulsing systems often caused somewhat cyclical heating and cooling due to the relatively long high-fire time and sometimes exceedingly long minimum-fire times.

After 35 columns over 10 years with Ceramic Industry, I have covered a lot of territory, ranging from efficiency to temperature uniformity. Along the way, I’ve outlined a range of combustion systems and operational techniques. Kiln technology advancement seems to be more of a step-change than a gradual evolution, and the step improvement that has occurred over the last 15 years has been related to pulse firing and control.

The initial pulsing systems used in this country were generally based on the idea of firing each burner for a fixed amount of time-say 8-12 seconds-and then forcing the burner to low-fire for a variable amount of time. While these types of systems worked adequately, they often caused somewhat cyclical heating and cooling due to the relatively long high-fire time and sometimes exceedingly long minimum-fire times.

The DataPaq curve shown in Figure 1 illustrates the cyclical heating associated with this type of pulsing logic. At the start of the cycle, no pulsing occurred at all because the kiln temperature was above set point. As the kiln approached 350-400°F, pulsing initiated and cyclical heating and cooling occurred in products set close to the burners.

Figure 2. Smoother heating occurs when the high-fire time is kept shorter.

As the system designs evolved, it became obvious that shorter bursts of energy, applied more frequently, would provide a smoother heating characteristic. Additionally, to obtain the best control, particularly near the beginning of the cycle, it was helpful to adjust the burners for the lowest possible minimum firing rate. In this way, almost all of the energy is applied to the kiln process at high fire, ensuring the best possible circulation.

The smoother heating shown in Figure 2 occurred because the high-fire time was much shorter. The burner fired in pulses as short as a half-second, and pulsing occurred regularly every 8-10 seconds. The combination of short periods of input and frequent pulsing allowed for smooth temperature control with all of the benefits of pulse firing, including the minimum use of required excess air, which translates to better efficiency, and reduced NO_x formation due to reduced excess air and high-velocity entrainment, which lowers the flame temperature.

As you upgrade your kilns or purchase new kilns, be aware that while pulse firing is the best current combustion solution, the details of the pulsing can make a huge difference in ultimate performance and efficiency.

Any views or opinions expressed in this column are those of the author and do not represent those of Ceramic Industry, its staff, Editorial Advisory Board or BNP Media.